Threaded-fastener wrenching structure, threaded fastener and wrenching tool

ABSTRACT

A threaded fastener capable of being reduced in size and weight. A pair of side wall surfaces of each of six radially outwardly projecting, torque transmitting portions lie on straight lines passing through the center line in a cross section perpendicular to the center line so as to transmit torque through the side wall surface, whereby the direction of a surface orthogonal force N applied to a force-applied point P by a wrenching tool coincides with the direction of an effective wrenching force F. This arrangement reduces an operation force required for rotating the wrenching tool and accordingly the load acting on each of the torque transmitting portions, in comparison with a conventional hexagonal head. That is, it is possible to reduce the diameter of the head portion of the threaded fastener and the size and weight of the head portion, while avoiding deformation of the side wall surfaces through which the torque is transmitted.

TECHNICAL FIELD

The present invention relates to a threaded-fastener wrenchingstructure, and more particularly to such a threaded-fastener wrenchingstructure which leads to reduction in size and weight of a threadedfastener.

BACKGROUND ART

Conventionally, a threaded fastener having a hexagonal head is generallywidely used. In a wrenching operation of such a threaded fastener havingthe hexagonal head, however, a driving angle α (see FIG. 10) is 60°,namely, there is generated a component of force which component is notdirected for wrenching the threaded fastener. It is therefore difficultthat the hexagonal head is made compact or reduced in its diameter andaxial height. As shown in FIG. 10, the driving angle α, which is 60° incase of the hexagonal head 100, corresponds to an angle between thedirection of a surface orthogonal force N and the direction of aneffective wrenching force F, wherein the surface orthogonal force Ncorresponds to a force applied to a force-applied point Q (correspondingto a vertex of the hexagon) while the effective wrenching force Fcorresponds to a force effective for wrenching the threaded fastener. Awrenching torque T is expressed by an expression (1) where “F” and “r”represent the effective wrenching force and a radius of the hexagonalhead, respectively. In this instance, as is apparent from an expression(2), the surface orthogonal force N applied to the force-applied point Qis twice as large as the effective wrenching force F. Meanwhile, anexpansive reaction force E, whose direction coincides with a radialdirection of the hexagonal head, is about 1.7 times as large as theeffective wrenching force F. If the radius r of the hexagonal headrequires to be reduced without reduction in the wrenching torque T, theeffective wrenching force F has to be increased inversely with thereduction of the radius r. For increasing the effective wrenching forceF, the surface orthogonal force N and the expansive reaction force Ehave to be increased. That is, it will be necessary to increase anoperation force for rotating the wrenching tool, thereby causing a riskof eroding the force-applied point Q, i.e., a corner portion of thehexagonal head 100.T=6×F×r  (1)N=F/cos 60°=2F  (2)E=F×tan 60°≈1.7F  (3)

With respect to “Hexagonal head bolt” of JIS B1180 and “Hexagonal headbolt with washer flange” of JIS B1189, their dimensions s, e, k asindicated in FIGS. 11 and 12 are specifically defined in relation with anominal diameter of thread as shown in FIG. 13. The dimension srepresents a diameter of a circle inscribed in the contour of thehexagonal head. The dimension e represents a diameter of a circlecircumscribed about the hexagonal contour. The dimension k represents aheight of the hexagonal head. A ratio of the circumscribed circlediameter e with respect to the nominal thread diameter d is representedby “e/d”. A ratio of the height k of the hexagonal head with respect tothe nominal thread diameter d is represented by “k/d”. As is apparentfrom FIG. 13, the circumscribed circle diameter e is not smaller than1.55d, and the height k of the hexagonal head is not smaller than 0.6d.

The present invention was made under the above-described background withobject of providing a threaded-fastener wrenching structure which makesit possible to reduce the size and weight of a threaded fastener,without reducing the wrenching torque applied to the threaded fastener.

DISCLOSURE OF INVENTION

For achieving the object, a first invention is a threaded-fastenerwrenching structure comprising: a fitting hole which is provided in oneof a threaded fastener and a wrenching tool for wrenching the threadedfastener; and a fitting protrusion which is provided in the other of thethreaded fastener and the wrenching tool and which is to be brought intofitting engagement with the fitting hole; wherein each of the fittinghole and protrusion has a contour having a plurality of torquetransmitting portions which are equi-angularly spaced apart from eachother about a center line of the each of the fitting hole and protrusionand which project outwardly in a radial direction of the each of thefitting hole and protrusion, so that a wrenching torque is transmittedthrough the torque transmitting portions to the threaded fastener, as aresult of rotation of the wrenching tool about the center line when thefitting hole and protrusion are held in fitting engagement with eachother, the threaded-fastener wrenching structure being characterized inthat (a) each of the torque transmitting portions has an engagingportion through which the wrenching torque is transmitted to thethreaded fastener, wherein a tangent line tangent to the engagingportion substantially coincides with a straight line passing through thecenter line in a cross section perpendicular to the center line, andthat (b) each of the torque transmitting portions has a radially outerportion which has, in a plan view thereof, a fan shape whose center isthe center line, wherein a portion of the contour between eachcircumferentially adjacent pair of the torque transmitting portionsdefines a pentagonal groove or a U-shaped groove having a smoothlycurved bottom surface.

A second invention is a threaded-fastener wrenching structurecomprising: a fitting hole which is provided in one of a threadedfastener and a wrenching tool for wrenching the threaded fastener; and afitting protrusion which is provided in the other of the threadedfastener and the wrenching tool and which is to be brought into fittingengagement with the fitting hole, wherein each of the fitting hole andprotrusion has a contour having a plurality of torque transmittingportions which are equi-angularly spaced apart from each other about acenter line of the each of the fitting hole and protrusion and whichproject outwardly in a radial direction of the each of the fitting holeand protrusion, so that a wrenching torque is transmitted through thetorque transmitting portions to the threaded fastener, as a result ofrotation of the wrenching tool about the center line when the fittinghole and protrusion are held in fitting engagement with each other, thethreaded-fastener wrenching structure being characterized in that (a)each of the torque transmitting portions has an engaging portion throughwhich the wrenching torque is transmitted to the threaded fastener,wherein a tangent line tangent to the engaging portion substantiallycoincides with a straight line passing through the center line in across section perpendicular to the center line, that (b) the threadedfastener has an external thread, that the fitting protrusion is providedin the threaded fastener, and that the fitting protrusion has a maximumdiameter Dmax, a minimum diameter Dmin and a height k as measured in adirection parallel with the center line such that the maximum diameterDmax, the minimum diameter Dmin and the height k satisfy the followingexpressions (4), (5), (6) which represent relationships with a majordiameter d of the external thread:Dmax≦1.5d  (4)1.1d<Dmin  (5)0.3d≦k<0.45d  (6)

A third invention is a threaded-fastener wrenching structure comprising:a fitting hole which is provided in one of a threaded fastener and awrenching tool for wrenching the threaded fastener; and a fittingprotrusion which is provided in the other of the threaded fastener andthe wrenching tool and which is to be brought into fitting engagementwith the fitting hole, wherein each of the fitting hole and protrusionhas a contour having a plurality of torque transmitting portions whichare equi-angularly spaced apart from each other about a center line ofthe each of the fitting hole and protrusion and which project outwardlyin a radial direction of the each of the fitting hole and protrusion, sothat a wrenching torque is transmitted through the torque transmittingportions to the threaded fastener, as a result of rotation of thewrenching tool about the center line when the fitting hole andprotrusion are held in fitting engagement with each other, thethreaded-fastener wrenching structure being characterized in that (a)each of the torque transmitting portions has an engaging portion throughwhich the wrenching torque is transmitted to the threaded fastener,wherein a tangent line tangent to the engaging portion substantiallycoincides with a straight line passing through the center line in across section perpendicular to the center line, that (b) the threadedfastener has an external thread, that the fitting protrusion is providedin the threaded fastener, and that the fitting protrusion has a maximumdiameter Dmax and a minimum diameter Dmin, both of which are smallerthan a major diameter d of the external thread.

The tangent line tangent to the engaging portion substantially coincideswith the straight line passing through the center line. This means thatthe intersection angle between these lines is not larger than ±3°. Theengaging portion may be provided by at least a portion of each of thetorque transmitting portions. It is ideal that the tangent line and thestraight line passing through the center line completely coincide witheach other. It is preferable that the intersection angle between theselines is not larger than ±1°.

A seventh invention is, in the threaded-fastener wrenching structuredefined in any one of the first through third inventions, characterizedin that the engaging portion is a flat surface having a predetermineddimension as measured in a direction of the tangent line.

An eighth invention relates to a threaded fastener, and is characterizedby being provided with the fitting hole or the fitting protrusiondefined in the first invention.

A ninth invention relates to a threaded fastener, and is characterizedby having an external thread and being provided with the fittingprotrusion defined in the second or third invention.

A tenth invention relates to a wrenching tool, and is characterized bybeing provided with the fitting hole or the fitting protrusion definedin the first invention.

In the threaded-fastener wrenching structure, each of the torquetransmitting portions has the engaging portion through which thewrenching torque is transmitted to the threaded fastener, wherein thetangent line tangent to the engaging portion substantially coincideswith the straight line passing through the center line in the crosssection perpendicular to the center line. In this arrangement, theabove-described driving angle α is substantially 0°, namely, thedirections of the surface orthogonal force N and the effective wrenchingforce F substantially coincide with each other, while the expansivereaction force E is substantially 0°. Therefore, a required amount ofthe surface orthogonal force N is smaller than that in the case of thehexagonal head shown in FIG. 10, thereby making it possible to reducethe operation force required for rotating the wrenching tool andaccordingly the load acting on each of the torque transmitting portions.That is, it is possible to reduce the diameter of the fitting hole orprotrusion without reducing the wrenching torque, while avoidingdeformation of the engaging portion. Thus, the threaded fastener or thewrenching tool can be made with reduction in its size and weight.

In the seventh invention, there is provided the flat surface having apredetermined dimension as measured in the direction of the tangentline, namely, there is provided the engaging portion which is flat andextends in a straight line passing substantially through the centerline. Therefore, the effective force F is distributed over the flatsurface, leading to reduction in a load per unit area of the surface.Thus, the threaded fastener or the wrenching tool can be made withfurther reduction in its size and weight.

In the second invention, the fitting protrusion provided in the threadedfastener having the external thread has the maximum diameter Dmax, theminimum diameter Dmin and the height k which satisfy the above-describedexpressions (4), (5), (6). Therefore, the size of the fitting protrusionin relation with the major diameter d of the external thread is reducedin comparison with a conventional hexagonal head, while deformation orfracture of each torque transmitting portion is avoided. Thus, thethreaded fastener is made with reduction in its size and weight.

In the threaded fastener of the eighth invention and the wrenching toolof the tenth invention, it is possible to obtain substantially the sameeffect as in the above-described first invention. In the threadedfastener of the ninth invention, it is possible to obtain substantiallythe same effect as in the second or third invention.

In the threaded-fastener wrenching structure, it would be common thatthe threaded fastener such as a hexagonal head bolt is provided with thefitting protrusion while the wrenching tool is provided with the fittinghole. However, the structure may be adapted such that the threadedfastener is provided with the fitting hole (recess) while the wrenchingtool is provided with the fitting protrusion (blade). Although each ofthe fitting hole and protrusion may be formed to be substantiallyparallel with the axial direction, each of the fitting hole andprotrusion may be adapted to include an inclined portion or a chamferedportion which are included with respect to the center line, in theinterest of facilitating its manufacture with forging process andfacilitating its fitting engagement in the wrenching operation.

In general, the number of the outwardly projecting, torque transmittingportions in each of the fitting hole and protrusion is appropriatelyfive or six. However, the number may be smaller than five or larger thansix.

The engaging portion, whose tangent line substantially coincides withthe straight line passing through the center line in the cross sectionperpendicular to the center line, is preferably provided by the flatsurface having a predetermined dimension as measured in the direction ofthe tangent line, as in the second invention. It is preferable that theengaging portion of each of the torque transmitting portions is providedby a pair of side wall surfaces each of which has a cross sectionalshape represented by a straight line extending toward the center lineaway from the radially outer portion. However, each of the side wallsurfaces does not have to be flat, but may be curved to have an arcuateshape in its cross section such that a line tangent to a portion of theside wall surface substantially coincides with the straight line passingthrough the center line.

In the third invention, the minimum diameter Dmin and even the maximumdiameter Dmax are smaller than the major diameter d. In such a case, theheight k may be adapted, as needed, to be not smaller than 0.45d suchthat the fitting protrusion is given a predetermined degree of fracturestrength.

In the second invention, the maximum diameter Dmax is adapted to be notlarger than 1.5d and the height k is adapted to be smaller than 0.45d,for the purpose of making these dimensions smaller than those of aconventional hexagonal head bolt. The height k is adapted to be notsmaller 0.3d, for the purpose of making the fracture strength of thehead portion, i.e., the fitting protrusion larger than the fracturestrength of the externally threaded portion when the threaded fasteneris wrenched.

Further, preferably, the fitting protrusion is provided in the threadedfastener, wherein it is desirable that a central angle θ1 of each of thetorque transmitting portions and a central angle θ2 of a portion of thecontour between each adjacent pair of the torque transmitting portionssatisfy the expression (7) described below. As a material of thewrenching tool, in “SocketWrench-Socket” of JIS B4636, for example, itis prescribed that the tool should be formed of SCM435 of JIS G4105 orits equivalent. The SCM435 belongs to strength grade 10.9, and hastensile strength of about 1040 N/mm². Meanwhile, it is common that thethreaded fastener is formed of a material belonging to strength grade8.8 and having tensile strength of 800 N/mm². Where the threadedfastener is constructed such that the expression (7) is satisfied, thetorque transmitting portions of the threaded fastener are prevented frombeing fractured. It is noted that the above-described term “strengthgrade”, which is used in the following description, is defined in“Mechanical properties of steel bolts and screws” of JIS B1051.1.3≦θ1/θ2≦1.4  (7)

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a set of views showing a threaded fastener which is oneembodiment of the present invention, wherein (a) is a plan view as seenfrom a head portion of the threaded fastener, while (b) is a crosssectional view taken along line (b)—(b) of (a).

FIG. 2 is a set of views showing a wrenching tool used for wrenching thethreaded fastener of FIG. 1, wherein (a) is a bottom plan view as seenfrom a socket portion of the wrenching tool, while (b) is a front viewas seen in a direction perpendicular to the center line S.

FIG. 3 is a table indicating an example of ratio (τ_(B)/Rm) of ashearing strength τ_(B) to a tensile strength Rm, in each strength gradeof material for the threaded fastener.

FIG. 4 is a table indicating a minimum height kmin, in each strengthgrade of material, which was obtained by using the ratio (τ_(B)/Rm) ofFIG. 3, and also a ratio (kmin/d) in the strength grade 12.9, by way ofexample.

FIG. 5 is a set of views explaining another embodiment of the invention,and corresponding to FIG. 1.

FIG. 6 is a set of views explaining still another embodiment of theinvention, and corresponding to FIG. 1.

FIG. 7 is a set of views explaining another embodiment of the invention,and corresponding to FIG. 1.

FIG. 8 is a set of views explaining still another embodiment of theinvention, and corresponding to FIG. 1.

FIG. 9 is a set of views explaining still another embodiment of theinvention, wherein (a) is a plan view as seen from its head portion,while (b) is a front view as seen in a direction perpendicular to itscenter line.

FIG. 10 is a view showing a hexagonal head of a conventional hexagonalhead bolt.

FIG. 11 is a set of views showing dimensions s, e, k, d in “Hexagonalhead bolt” of JIS B1180.

FIG. 12 is a set of view showing dimensions s, e, k, d in “Hexagonalhead bolt with flange washer” of JIS B1189.

FIG. 13 is a table indicating examples of the dimensions s, e, k shownin FIGS. 11 and 12, and also “e/d” and “k/d” obtained from values of thedimensions s, e, k.

BEST MODE FOR CARRYING OUT THE INVENTION

There will be described in detail embodiments of the present invention,with reference to the drawings.

FIG. 1 is a set of views showing a threaded fastener 10 which is oneembodiment of the present invention, wherein (a) is a plan view as seenfrom a head portion 12 of the threaded fastener 10, while (b) is a crosssectional view taken along line (b)—(b) of (a). The threaded fastener 10has an external thread 14 which is integrally formed such that theexternal thread 14 is adjacent to the head portion 12 and is concentricwith a center line O. FIG. 2 shows a wrenching tool 30 having a socketportion 32 which is to be brought into fitting engagement with the headportion 12. The threaded fastener 10 is rotated about the center line Oby the wrenching tool 30, so as to be tightened or loosened. The headportion 12 corresponds to a fitting protrusion, and has a contour havingsix torque transmitting portions 16 which projects radially outwardlyand which are equi-angularly spaced apart from each other at an angularinterval of 60° about the center line O. As shown in the plan view ofFIG. 1 (a), each of the torque transmitting portions 16 has an outerwall surface which has, in its cross section, an arcuate shape whosecenter coincides with the center line O. Each of the torque transmittingportions 16 further has a pair of side wall surfaces 18 corresponding toan engaging portion. Each of the side wall surfaces 18 is, in its crosssection, represented by a straight line extending from the outer wallsurface toward the center line O. An intersection angle defined by thisstraight line and a straight line passing through the center line O issubstantially 0°. In the present embodiment, this intersection angle isnot larger than ±1°. The outer wall surface of each transmitting portion16 is provided by a part-cylindrical surface which is parallel with thecenter line O. The side wall surfaces 18 of each transmitting portion 16are provided by flat surfaces which are parallel with the center line O.The side wall surfaces 18 of each adjacent pair of the torquetransmitting portions 16 are connected to each other via a pair ofinclined flat surfaces 20 which are inwardly inclined symmetrically witheach other, so that a portion of the contour between each adjacent pairof the torque transmitting portions 16 defines a groove 22 which has apentagonal shape in its cross section. It is noted that the shaperepresented by the plan view of FIG. 1( a) corresponds to the crosssectional shape in a plane perpendicular to the center line O.

The head portion 12 has a maximum diameter Dmax, a minimum diameter Dminand a height k such that the maximum diameter Dmax, the minimum diameterDmin and the height k satisfy the above-described expressions (4), (5),(6) which represent their relationships with a major diameter d of theexternal thread 14. Each of the torque transmitting portions 16 subtendsa central angle θ1, while the portion of the contour between eachadjacent pair of the torque transmitting portions 16 subtends a centralangle θ2. The central angles θ1, θ2 as measured about the center line Osatisfy the above-described expression (7). In FIG. 1, Dmax=1.5 d,Dmin=1.1 d, k=0.35 d, θ1/θ2≈1.4. It is noted that two-dot chain lines inFIG. 1 represents a contour of the hexagonal head 100 of the hexagonalhead bolt defined by JIS B1180, for comparison with the contour of thehead portion 12 of the threaded fastener 10.

On the other hand, the socket portion 32 of the wrenching tool 30 isprovided by a cylindrical member having a fitting hole 34 whose bottomis defined by a bottom wall of the cylindrical member. FIG. 2( a) is abottom plan view as seen from the socket portion 32, while FIG. 2( b) isa front view as seen in the direction perpendicular to a center line S.The fitting hole 34 has a contour having six torque transmittingportions 36 which project radially outwardly and which areequi-angularly spaced apart from each other at an angular interval of60° about the center line S. In the bottom plan view of FIG. 2( a), eachof the torque transmitting portions 36 has a fan shape, and has theengaging portion in the form of a pair of side wall surfaces 38 each ofwhich is, in its cross section, represented by a straight line extendingin a direction away from the radially outer portion toward the centerline S. The fitting hole 34 has dimensions each of which is adapted tobe slightly larger than a corresponding one of dimensions of the headportion 12, so that the fitting hole 34 can be fitted on the headportion 12. In a wrenching operation in which the threaded fastener 10is wrenched by the wrenching tool 30, the socket portion 32 isconcentrically fitted on the head portion 12 such that the torquetransmitting portions 16 of the head portion 12 are received in therespective torque transmitting portions 36 of the fitting hole 34. Thesocket portion 32 of the wrenching tool 30 is then rotated about thecenter line S, so that the torque is transmitted to the threadedfastener 10 through the side wall surfaces 38, 18.

In a threaded-fastener wrenching structure constituted by the threadedfastener 10 and the wrenching tool 30 which are constructed as describedabove, each of the six torque transmitting portions 16, 36 has the sidewall surfaces 18, 38 through which the wrenching torque is transmittedto the threaded fastener 10. Since each of the side wall surfaces 18, 38lies on a straight line passing through the center line O, S in thecross section perpendicular to the center lines O, S, the direction of asurface orthogonal force N applied by the wrenching tool 30 to aforce-applied point Q coincides with a circumferential direction aboutthe center line O, namely, the direction of an effective wrenching forceF which is effective for wrenching the fastener 10, while an expansivecomponent force E becomes substantially zero. Therefore, a requiredamount of the surface orthogonal force N becomes smaller than that inthe case of the conventional hexagonal head 100, thereby making itpossible to reduce the operation force required for rotating thewrenching tool 30 and accordingly reduce the load acting on each of thetorque transmitting portions 16, 36. That is, it is possible to reducethe diameter of the head portion 12 of the fastener 10 and accordinglyreduce the size and weight of the head portion 12 of the fastener 10,while assuring a predetermined amount of the effective wrenching torqueand avoiding deformation of the side wall surfaces 18, 38.

Particularly, in the present embodiment in which the flat side wallsurfaces 18, 38 each provided to lie on the straight line passingthrough the center lines O, S, the effective wrenching force F isdistributed over the flat side wall surface, leading to reduction in aload acting on an unit area of the surface. Thus, the head portion 12 ofthe threaded fastener 10 can be made with further reduction in its sizeand weight.

That is, in the present embodiment in which the head portion 12 of thethreaded fastener 10 has the maximum diameter Dmax of 1.5d and theheight k of 0.35d, the size of the head portion 12 is considerablysmaller than the conventional hexagonal head 100 as is apparent fromFIG. 1, whereby the fastener 10 can be made with reduction in its sizeand weight.

In the study of the height k of the head portion 12, the maximum tensilestrength Fmax of the external thread 14 and the maximum tensile strengthFmax of the head portion 12 are respectively expressed by the followingexpressions (8), (9) in which tensile strength Rm of the material of thethreaded fastener, effective cross sectional area A(s) of the externalthread 14, shearing strength 1B of the material of the threaded fastenerand shearing area Ask (=k×π×d) of the head portion 12 are used. Sincethe maximum tensile strength Fmax of the head portion 12 requires to belarger than the maximum tensile strength Fmax of the external thread 14,namely, since Fmax (head portion)>Fmax (thread), the followingexpression (10) is obtained. The minimum height Kmin is expressed by thefollowing expression (11).Fmax (thread)=Rm×A(s)  (8)Fmax (head portion)=τ_(B) ×Ask  (9)τ_(B) ×k×π×d>Rm×A(s)  (10)$\begin{matrix}\begin{matrix}{{kmin} = {\left( {{Rm} \times {A(s)}} \right)/\left( {\tau_{B} \times \pi \times d} \right)}} \\{= {{A(s)}/\left\lbrack {\left( {\tau_{B}/{Rm}} \right) \times \pi \times d} \right\rbrack}}\end{matrix} & (11)\end{matrix}$

On the other hand, the ratio (τ_(B)/Rm) of the shearing strength τ_(B)to the tensile strength Rm of the material of the fastener is determinedin each strength grade, as shown in the table of FIG. 3, according to“Systematic calculation of high duty bolted joints” defined in VDI 2230of German Association of Engineers. The table of FIG. 4 shows theminimum height kmin obtained from the above-described expression (11),for each strength grade in the threaded fasteners 10 having differentdiameters d. The rightmost column of the table of FIG. 4 indicates theratio (kmin/d) in the strength grade 12.9. Therefore, the height k of0.35d as in the present embodiment provides a sufficient degree ofshearing strength even in case of the strength grade 12.9. In case ofthe strength grade 8.8 or 10.9, namely, in alleviated condition, asufficient degree of shearing strength is obtained even if the height kis about 0.3d.

In the present embodiment in which the angle θ1 of each torquetransmission portion 16 and the angle θ2 of each groove 22 satisfy therelationship expressed by the above-described expression (7), eachtorque transmitting portion 16 of the threaded fastener 10 isadvantageously prevented from being fractured. Where the material of thethreaded fastener 10 belongs to the strength grade 8.8 while thematerial of the wrenching tool 30 is, as defined in“SocketWrench-Socket” of JIS B4636, SCM435 of JIS G4105 which belongs tothe strength grade 10.9, namely, where the tensile strength of thematerial of the threaded fastener 10 is about 800 N/mm² while thetensile strength of the material of the wrenching tool 30 is about 1040N/mm², the fracture strength of the threaded fastener 10 is larger thanthat of the wrenching tool 30 if the expression (7) is satisfied.

Next, there will be explained other embodiments of the presentinvention. It is noted that the same reference numerals as used in theabove-described embodiment will be used to identify the substantiallysimilar portions, which will not be explained in detail.

FIG. 5 shows a threaded fastener 40 in which five torque transmittingportions 16 are equi-angularly spaced apart from each other about thecenter line O, wherein the dimensions Dmax, Dmin, k, θ1, θ2 are set tosatisfy the above-described expressions (4), (5), (6) and (7).

FIG. 6 shows a threaded fastener 42 in which a recess 44 is formed inthe head portion 12 of the threaded fastener 10. The recess 44 has sixgrooves corresponding to the torque transmitting portions 16. Thisthreaded fastener 42 can be wrenched by using two kinds of wrenchingtools. It is noted that the recess 44 is not essential for the presentinvention and does not have to have an engaging portion whichsubstantially coincides with a straight line passing through the centerline O.

FIGS. 7 and 8 show threaded fasteners 46, 48 in each of which thepentagonal grooves 22 are replaced by U-shaped grooves 52 each having asubstantially semi-circular-shaped bottom surface 50 that is curvedsmoothly from the flat side wall surfaces 18. Each of the grooves 52 haswidthwise opposite edges that are rounded. Each of the threadedfasteners 46, 48 provides the same effect as the threaded fasteners 10,40.

FIG. 9 shows a studbolt 60 to which the principle of the presentinvention is applied. The studbolt 60 includes a threaded portion 68which has a pair of external threads 64, 66, and a head portion 62 whichserves as the fitting protrusion and which is coaxial with the threadedportion 68. The head portion 62 has, in its plan view, the sameconfiguration as the above-described head portion 12. Thus, like thehead portion 12, the head portion 62 has a contour having the six torquetransmitting portions 16 each having the pair of side wall surfaces 18.However, either of the diameters Dmax, Dmin is smaller than the majordiameter d of the external threads 64, 66. In this embodiment, too,since the torque is transmitted through the flat side wall surfaces 18lying on the straight lines passing through the center line O in theplan view, namely, since the direction of the force (=surface orthogonalforce N) applied to each side wall surface 18 is the same as thedirection of the effective wrenching force F, each side wall surface 18is being prevented from being deformed, even if a required amount of thewrenching force F is increased due to the reduction of the size of thehead portion 62. It is noted that FIG. 9 (a) is the plan view as seenfrom the head portion 62 while FIG. 9 (b) is the front view as seen inthe direction perpendicular to the center line.

The embodiments of the present invention have been explained in detailwith reference to the drawings. However, each of the embodiments ismerely an embodied form, and the present invention can be embodied withvarious modifications and improvements on the basis of knowledge ofthose skilled in the art.

1. A threaded-fastener wrenching structure, comprising: a fitting holewhich is provided in one of a threaded fastener and a wrenching tool forwrenching said threaded fastener; and a fitting protrusion which isprovided in the other of said threaded fastener and said wrenching tooland which is to be brought into fitting engagement with said fittinghole; wherein: each of said fitting hole and protrusion has a contourhaving a plurality of torque transmitting portions which areequi-angularly spaced apart from each other about a center line of saideach of said fitting hole and protrusion and which project outwardly ina radial direction of said each of said fitting hole and protrusion, sothat a wrenching torque is transmitted through said torque transmittingportions to said threaded fastener, as a result of rotation of saidwrenching tool about said center-line when said fitting hole andprotrusion are held in fitting engagement with each other; each of saidtorque transmitting portions has an engaging portion through which thewrenching torque is transmitted to said threaded fastener, wherein atangent line tangent to said engaging portion substantially coincideswith a straight line passing through said center line in a cross sectionperpendicular to said center line; said threaded fastener has anexternal thread; said fitting protrusion is provided in said threadedfastener; and said fitting protrusion has a maximum diameter Dmax, aminimum diameter Dmin and a height k as measured in a direction parallelwith said center line such that said maximum diameter Dmax, said minimumdiameter Dmin and said height k satisfy the following expressions (4),(5), (6) which represent relationships with a major diameter d of saidexternal thread:Dmax≦1.5d  (4)1.1d<Dmin  (5)0.3d≦k<0.45d  (6).
 2. The threaded-fastener wrenching structureaccording to claim 1, wherein an intersection angle between said tangentline and said straight line passing through said center line is no morethan +3° and no less than −3°.
 3. The threaded-fastener wrenchingstructure according to claim 1, wherein an intersection angle betweensaid tangent line and said straight line passing through said centerline is no more than +1° and no less than −1°.
 4. The threaded-fastenerwrenching structure according to claim 1, wherein said plurality oftorque transmitting portions consist of five or six torque transmittingportions.
 5. The threaded-fastener wrenching structure according toclaim 1, wherein said engaging portion is a flat surface having apredetermined dimension as measured in a direction of said tangent line.6. A threaded fastener, having an external thread, and provided with thefitting protrusion defined in claim
 1. 7. The threaded-fastenerwrenching structure according to claim 1, wherein said fittingprotrusion is provided in said threaded fastener and a central angle θ1of each of said torque transmitting portions and a central angle θ2 of aportion of said contour between each adjacent pair of said torquetransmitting portions satisfy the following expression (7):1.3≦θ1/θ2≦1.4  (7).
 8. A threaded-fastener wrenching structure,comprising: a fitting hole which is provided in one of a threadedfastener and a wrenching tool for wrenching said threaded fastener; anda fitting protrusion which is provided in the other of said threadedfastener and said wrenching tool and which is to be brought into fittingengagement with said fitting hole; wherein: each of said fitting holeand protrusion has a contour having a plurality of torque transmittingportions which are equi-angularly spaced apart from each other about acenter line of said each of said fitting hole and protrusion and whichproject outwardly in a radial direction of said each of said fittinghole and protrusion, so that a wrenching torque is transmitted throughsaid torque transmitting portions to said threaded fastener, as a resultof rotation of said wrenching tool about said center line when saidfitting hole and protrusion are held in fitting engagement with eachother; each of said torque transmitting portions has: an engagingportion through which the wrenching torque is transmitted to saidthreaded fastener, wherein a tangent line tangent to said engagingportion substantially coincides with a straight line passing throughsaid center line in a cross section perpendicular to said center line;and a radial outer end surface which is provided by a part of acylindrical surface whose center is said center line; said threadedfastener has an external thread; said fitting protrusion is provided insaid threaded fastener; and said fitting protrusion has a maximumdiameter Dmax, a minimum diameter Dmin and a height k as measured in adirection parallel with said center line such that said maximum diameterDmax, said minimum diameter Dmin and said height k satisfy the followingexpressions (4), (5), (6) which represent relationships with a majordiameter d of said external thread:Dmax≦1.5d  (4)1.1d<Dmin  (5)0.3d≦k<0.45d  (6).
 9. The threaded-fastener wrenching structureaccording to claim 8, wherein a portion of said contour between eachcircumferentially adjacent pair of said torque transmitting portionsdefines a groove which has a pentagonal shape in a cross sectionperpendicular to said center line.
 10. The threaded-fastener wrenchingstructure according to claim 8, wherein a portion of said contourbetween each circumferentially adjacent pair of said torque transmittingportions defines a groove which has a U shape in a cross sectionperpendicular to said center line.